Transcript Radon 1

Radon, Part 1
GLY 4241 - Lecture 10
Fall, 2016
1
Radiation
• Radiation is one of many areas that have
become of concern to environmental
scientists
• Radiation comes from two sources
 Natural processes
 Man
2
Natural Radioactivity
• Two kinds
 Short lived nuclides formed by cosmic ray
interactions with the atmosphere
 Long lived isotopes that have persisted since
planetary formation
 May be concentrated by natural or anthropogenic
processes
3
Common Long-Lived Isotopes
• Uranium


235U
238U
• Thorium

232Th
• All decay to yield isotopes of lead
4
232Th
Decay Series
5
235U
Decay Series
6
238U
Decay Series
7
Properties of Radon
•
•
•
•
Colorless, odorless gas
Soluble in both water and organic solvents
Atomic number 86
Permissible Concentration
 0.4 x 10-2 picoCuries/cm3, or 4 pCi/liter
 World Health Organization now recommends 2.7
pCi/liter
• One curie is the number of disintegrations per
second of one gram of radium, or 3.7 x 1010
disintegrations per second
8
Radon Isotopes
9
Sievert
• Another unit in use is the sievert (symbol: Sv), named after
Rolf Sievert, a Swedish medical physicist
• It is the SI derived unit of dose equivalent, which attempts
to reflect the biological effects of radiation as opposed to
the physical aspects
• The worldwide average human exposure from natural
sources of radiation is 2.4 millisieverts (mSv) per year
• About half comes from the radioactive decay products
(mostly α-particles) of radon gas, with most of the
remaining natural exposure comes from cosmic rays and
terrestrial gamma-rays
10
Human Radiation Exposure
Source
Average annual exposure,
mSv/yr
Radon
1.2
Cosmic rays and terrestrial γ
rays
1.2
Frequent Flyer
0.5
Diagnostic medical procedures
0.4
Fallout from nuclear tests
0.005
Chernobyl accident (local
exposure)
0.002
Nuclear power production
0.0002
From Little, 2003
11
Radiation Exposure from Radon
• Based on data
from the United
Kingdom
• About half of
human radiation
exposure in the
UK is from radon
12
Isotopes of Concern
• Two isotopes are of particular concern, the the alpha
emitters 220Rn and 222Rn
• Alpha particles are the nuclei of 4He atoms
• They are heavy and are emitted with a great deal of energy
• Although they are not very penetrating, being stopped by a
piece of paper, they do a great deal of damage if they
disintegrate within a cell
• The alpha particles from 220Rn and 222Rn ionize about
250,000 and 180,000 atoms, respectively, per
distintegration
13
Beta Emitters
• Beta emitters cause less localized damage
• Beta particles are electrons
• They are negatively charged and are repelled by
the other electrons of ions and atoms
• A 6 Mev beta particle travels about 3 centimeters
in biological tissue
• Beta emitters are dangerous externally and
internally
14
Radon Nomenclature
• The isotope 220Rn is sometimes called
Thoron since it is a daughter of Thorium
• Radon is sometimes used to refer to the
isotope 222Rn alone, but this usage can be
confusing and will not be used in this
course
15
Biological Effects of Ionizing
Radiation
• A single ionization in a nucleic acid can alter the
genetic instructions for assembling the protein
constituents of the cell
• Certain kinds of alteration, or combinations of
alteration, are believed to be carcinogenic
• A well-known correspondence between radiation
exposure and cancer exists
• In biological tissue, an alpha particle travels about
0.05 millimeters before stopping
16
Relative Rn Risk
17
Assessing Risk
• “ ‘The risk estimates obtained in this study suggest that
cumulative [total] radon exposure in the residential environment
is significantly associated with lung cancer risk.’
• After adjusting for age, smoking, and education, and using
categorical radon exposure levels, a 15 year exposure at levels
equivalent to EPA's action level of 4 pCi/L yielded excess odds
of 0.50 i.e., an increase in lung cancer risk of 50% (95%
confidence interval: 0.004, 1.81) for total cases and excess odds
of 0.83 for cases with personal interviews i.e., an increase in
lung cancer risk of 83% (95% confidence interval: 0.11, 3.34).
• The higher risk found for cases with personal interviews vs. the
total cases most probably reflects the more accurate exposure
assessment obtained from interviews with cases vs. interviews
with relatives.”
18
Julia Harris Video
19
Radon Parental Material
• Rn is a daughter of uranium or thorium, and is
usually associated with rocks that contain
significant quantities of U and/or Th
• Both U and Th are lithophile elements
• Strongly lithophile elements prefer the silicate or
oxide phases over sulfide or metallic phases
• In the silicates and oxides, they exhibit ionic
bonding
20
Ionic Radii of Th and U
• All radii in nm
21
Uranium Minerals
•
•
•
•
•
Uraninite (UO2)
Carnotite (K2(UO2)2(VO4)2• 3 H2O)
Tyuyamunite (Ca(UO2)2(VO4)2 • 5-8½ H2O)
Torbernite (Cu(UO2)2(PO4)2 • 8-12 H2O)
Autunite (Ca(UO2)2(PO4)2 • 10-12 H2O)
22
Thorium Minerals
• Thorianite (ThO2)
• Thorite (ThSiO4)
23
Carnotite Associated with Wood
24
Carnotite in Shale
• A layer of yellow
Carnotite that is
sandwiched in
between layers of a
soft shale
• Anderson Mine,
Yavapai County,
Arizona, U.S.A.
25
Forsterite Hydrolysis
• Examples include the hydrolysis of the silicate
olivine (forsterite) to yield silicic acid and
magnesium ion
26
Plagioclase Feldspar Hydrolysis
• Hydrolsis reaction from albite to the clay
mineral kaolinite
27
Sulfide Minerals
• Pyrite, FeS2, is the major sulfide present
• Many minor elements are enriched in black
shales, including V, U, As, Sb, Mo, Cu, Ni,
Cd, Ag, Au and platinum group metals
• Concentration of U
 Correlates with increasing C in some smaples
 Also correlates with grain size
 And in phosphatic shales
28
Phosphoria
Formation
• The Western
Phosphate Field is a
350,000 km2 area of
the northern Rocky
Mountains
• Darkened areas
represent exposures
of the Phosphoria
Formation.
29
U in Coal
• Coal consists almost entirely of organic
hydrocarbons
 Organic geopolymers, modified by hydrolysis
and microbial degradation
 Microbes often convert sulfates to sulfides
 Some products are easily broken down
 Others, often porphyrins, are quite resistant
30
Metal Enrichment in Coal
• Coal is often rich in sulfur, uranium, and
vanadium
 Some coals have been used as ores of vanadium
• Radon levels in coal mines are often elevated
 Levels of 1650 Becquerels/m3 were found in a
Brazilian mine in operation since 1942
 This is about 11 times the maximum exposure level in
indoor air, and is higher than levels permitted in mines
in most countries
31
Thorium
• Does not show multiple oxidation states
• Found primarily in felsic igneous rocks and
the immediate weathering products of these
rocks
• It is usually unaltered by chemical
weathering, although physical weathering
may reduce the grain size
• Much uranium is also found in the same
environment.
32
Th and U Content in Selected Rocks
• All data are in parts per million (ppm)
33
Ra Concentrations in Selected Igneous
Rocks and Resulting Radioactivity
34
Ra Concentrations in Selected Non-Igneous
Rocks and Resulting Radioactivity
35
New
Hampshire
Radon
Levels
• Radon potential
defined on the
basis of lithology
and mineralogy
36